The present invention relates to a sealed alkaline storage battery using a negative electrode of a hydrogen absorbing alloy capable of electrochemically absorbing/desorbing hydrogen acting as an active material.
Recently, hydrogen absorbing alloys capable of electro-chemically absorbing/desorbing hydrogen acting as an active material have attracted attention for use as a negative electrode material having high energy density. High-capacity sealed storage batteries such as a sealed nickel-hydrogen storage battery and a closed manganese dioxide-hydrogen storage battery have been developed by combining this type negative electrodes and available positive electrodes such as a nickel positive electrode and a manganese dioxide positive electrode. In the past, this type negative electrodes for use in the sealed nickel-hydrogen storage battery and methods of producing this type negative electrodes have been proposed as follows.
(1) A method of producing a negative electrode comprising filling a nickel porous matter having a three-dimensional structure with hydrogen absorbing alloy powder together an alkali-resisting binding agent (Japanese Patent Unexamined Publication No. 53-38631).
(2) A method of producing a negative electrode comprising the steps of: applying hydrogen absorbing alloy powder to a supporting metal; and sintering the alloy powder in an atmosphere of an inert gas (Japanese Patent Unexamined Publication No. 62-278754).
(3) A method of producing a negative electrode comprising the steps of: kneading hydrogen absorbing alloy powder and polytetrafluoroethylene (hereinafter referred to as "PTFE") powder or its aqueous solution dispersion while applying shearing stress thereto; pressing the kneaded matter to prepare a sheet; and sticking the sheet to a nickel net or the like (Japanese Patent Unexamined Publication No. 60-136162).
(4) A method in which at least one portion of a negative electrode formed of a hydrogen absorbing alloy is provided with a hydrophobic layer (Japanese Patent Unexamined Publication No. 61-118963).
The negative electrodes produced by the aforementioned methods have a disadvantage in that short-time (about an hour) charging required in various types of portable apparatus and the like is difficult or in that the reliability of the battery may deteriorate with repetition of charging/discharging though short-time (rapid) charging may be made in the early stage where the battery is just constructed. When the prior art negative electrodes formed by the above methods (1) and (2) are subject to relatively slow charging with respect to which the time required for perfect charging is within a range of from about 4.5 hours to about 16 hours, there arises no problem particularly awaiting solution. When the prior art negative electrodes produced by the above methods (1) and (2) are subject to rapid charging, however, the internal gas pressure of the battery increases in an overcharging period so that a safety vent (which is, in general, actuated by the battery inner pressure of 10 to 15 kg/cm.sup.2 in the same manner as an ordinary nickel-cadmium battery) is actuated so that the alkaline electrolyte of the battery leaks to constitute an obstacle to various battery characteristics and safety. Accordingly, in the prior art negative electrodes produced by the above methods (1) and (2), short-time charging is impossible.
In the prior art negative electrode produced by the above method (3), the respective sheets composed of fiber-like fluororesin and hydrogen absorbing alloy powder are located on the opposite sides of the nickel net respectively. In the configuration of the negative electrode, the fiber-like fluororesin gives suitable hydrophobic property to the surfaces of the hydrogen absorbing alloy powders to improve gas absorption ability in an overcharging period to thereby make it possible to perform short-time charging in the early stage where the battery is just constructed. It is however very difficult to prepare reproducibly similar sheets by the aforementioned steps of; kneading hydrogen absorbing alloy powder and PTFE powder while applying shearing stress thereto; and pressing the mulled matter. Accordingly, the internal gas pressure of the battery in a short-time charging period varies widely and sometimes exceeds 20 kg/cm.sup.2. Furthermore, the negative electrode is inferior in mechanical strength because the sheet formed by tangling the alloy particles with the fluororesin fibers is stuck to the nickel net. Accordingly, the alloy and the fluororesin are subject to expansion/contraction due to repetition of charging/discharging and changes in temperature, resulting in deterioration in the negative electrode. Accordingly, the prior art negative electrode produced by the above method (3) has a problem in that the reliability of the battery deteriorates with repetition of charging/discharging.
The proposal by the above method (4) is that a hydrophobic layer is provided in a portion of a negative electrode to thereby improve gas absorption capacity in an overcharging period. Although short-time charging is possible in this case, there arises a problem in that the hydrophobic layer provided in the negative electrode is made to come off by the expansion/contraction of the alloy due to repetition of charge/discharge or by a gas generated, so that the internal gas pressure of the battery increases with repetition of charging/discharging cycles.